Reinforcing resins are typically phenolic novolac resins that are used to increase the hardness and low-strain modulus (stiffness) of rubber compounds. Reinforcing resins can be used with a methylene donor (e.g., hexamethoxymethylmelamine (HMMM) or hexamethylenetetramine (HEXA/HMT)) in a rubber composition so that they cure while the rubber compound is being cured by sulfur.
Phenol-formaldehyde resins and phenol-formaldehyde resins modified with fatty acids (e.g., a phenol-formaldehyde resin modified with a distilled tall oil or cashew nut shell liquid and subjected to a heat treatment) have been used as reinforcing resins. These reinforcing resins, when used in a rubber composition, can increase the modulus at low percentage strain and dynamic strain of the rubber compound. When using reinforcing resins, it is a desirable to increase the hardness of the rubber compound. However, the cure characteristics (scorch or early onset of cure) of the rubber compounds when incorporating these reinforcing resins may be negatively impacted. Moreover, adding these reinforcing resins typically increases mixing viscosity of the rubber composition, making the formulation process for the rubber compounds more difficult and more energy consuming and time-consuming.
The conventional process of preparing reinforcing resins by reacting alkylphenol and/or phenol directly with aldehyde can leave a significant amount of residual monomers in the resins. However, it is desirable to reduce the residual phenolic monomer(s) in the resulting reinforcing resins for environmental reasons as well as for reducing the toxicity of the resins. For example, alkylphenols such as para-tertiary octylphenol (PTOP) are the object of environmental and toxicological studies (risk assessment studies), and it is typically desirable to limit their use in the free monomeric form in novolac alkylphenol resins. Decreasing free alkylphenol monomers in resins also permits a simpler manipulation of resins by decreasing the vapor emissions of alkylphenol monomers in factories or in atmospheric discharges when they are used, particularly in rubber applications when vulcanization is carried out at an elevated temperature.
Although the free residual monomer(s) in the resins can be distilled out at the end of the resin synthesis, this conventional distillation process presents several disadvantages. First, the distillation of the free monomer(s) of alkylphenol or phenol brings a non-trivial loss of primary material and requires recycling or treatment of the distilled alkylphenol or phenol. Second, the process is difficult to implement at an industrial level for an alkylphenol, such as PTOP, which has very high boiling point, and thus additional equipment, such as a powerful vacuum and/or elevated temperatures are used in the reactor. Moreover, alkylphenols (e.g., para-tertiary butylphenol (PTBP) and PTOP) are typically solid compounds and can crystallize in the piping, which requires reheating to prevent plugs.
Therefore, there remains a need to develop a process for preparing reinforcing resins, which, when used in a rubber composition, not only provides increased modulus and hardness of the rubber composition, but also improves the cure characteristics and reduces the mixing viscosity of the rubber composition. There is also a need to develop a process for preparing reinforcing resins that can reduce free residual monomer content in the resulting resins. This invention answers those needs.